The deployment of optical fiber in the telephone access network has recently been accelerating. One of the more popular architectures uses a passive optical network (PON). The current generation of PONs is intended to be upgradeable to broadband service delivery. Any such upgrade must deliver transmission rates suitable for a realistic set of target services at the lowest possible cost. One such cost-effective broadband PON architecture that incorporates coarse wavelength division multiplexing (WDM) to separate upstream and downstream traffic, time division multiplexing (TDM), and subcarrier multiple access (SCMA) is described in our article "Demonstration of a Cost-Effective, Broadband Passive Optical Network System," IEEE Photonics Technology Letters, Vol. 6, No. 4, pp. 575-578, April 1994, and U.S. Pat. No. 5,311,344 which issued on May 10, 1994 to T. H. Wood and P. P. Bohn.
SCMA transmission systems are attractive for several reasons. They use simple radio techniques, require no synchronization, can be designed to make little demand on laser linearity, and can be scaled to very high bit rates. However, SCMA systems can suffer from optical beat interference (OBI), which adversely effects system performance by increasing the noise transmitted on the data channels. This problem is exacerbated when laser transmitters are employed since the narrow optical spectrum of the lasers increases the impairment that the system experiences from OBI.
There have been a number of experimental and theoretical attempts to broaden the optical spectra to limit the penalty caused by OBI. A broader optical spectrum leads to a broader spectrum for the beat note, thus resulting in less optical beat noise within a given electrical bandwidth. Attempts to broaden the optical spectrum of lasers include the use of self-pulsating lasers (see R. J. S. Bates et al., Electron. Lett. 27, 1014, 1991) and modulating lasers with a modulation index greater than one (see T. H. Wood et al., J. Lightwave Technol. 11, 1632, 1993). A theoretical investigation has described the use of external modulators to broaden the spectrum through optical phase modulation. (see M. N. Banat et al., J. Lightwave Technol. 12, 1863, 1994). All of these techniques have significant limitations. Self-pulsating lasers are not readily available at optical communications wavelengths, and in any case their reliability is unknown. By using an optical modulation index greater than one the power consumption increases. Moreover, the power of the transmitting lasers may still require adjustment to ensure that the powers received from each laser by the optical detector are all nearly equal. Moreover, the external modulators are prohibitively expensive.
Light emitting diodes (LED) would otherwise be an attractive solution to the problem of OBI except that too little of their power can be coupled into a single mode fiber. The inherently broad optical spectrum of an LED should make OBI penalties negligible, and LED technology is well-established and reliable. For example, Bellcore suggests a loss budget of 24.4 dB for a PON with a 16-way split (see Bellcore reference TR-NWT-000909 entitled "Generic Requirements and Objectives for Fiber in the Loop Systems,", December, 1991, p. 2-25). Such PONs are required to operate with a bit error rate below 10.sup.-9. The received power level required to achieve this bit error rate level will depend on the subcarrier modulation format and the bit rate. For example, the inventors have determined that a 3 Mb/s frequency-shift-keyed subcarrier data stream has been found to require a received power level of -38 dBm. Consequently, the power launched into the fiber by the LED must be greater than -13.6 dBm. LEDs must be able to provide this power level over the lifetime of the PON at temperatures that may be as high as 85.degree. C. No available LEDs can meet such a demanding requirement. Moreover, as lightwave transmission systems increase their bit rates, the power requirements will also increase. Accordingly, the limited power of LEDs becomes even more problematic.